It's not necessarily a principle, but if there were a Goods card that attached to a condition like Ol' Howard is, after his grifter ability is used, such hypothetical card would need to specify what it can attach or cannot attach to.
The rulebook part that you're referencing to, is referencing to the good cards that can attach to non-dude cards (Deeds for example), like Auto-Gatling, Legal Instruments, Blight Serum, or Fate Dispenser, to name a few.
They cannot. There's no such principle like that in Doomtown.
To your other question, Monte Bank cannot be attached to Ol' Howard after Ol' Howard attaches to a deed. At that point, Ol' Howard is no longer a dude, and permanently becomes a Condition while Ol' Howard remains in play. This does mean that other cards that interact with cards attached to locations (Idol of Tlazolteotl,or All or Nothing for example), or Conditions, can interact with Ol' Howard. Lastly, Ol' Howard retains whatever keywords he has: Grifter, and Abomination.
For the future, I'd encourage breaking up the questions that you have into multiple posts. While I encourage and welcome your questions, it's easier to track questions and responses that way.